Marie,
You may already know this, but some avocados naturally are biennial fruiting trees -- meaning every other year they bear heavier than the year in-between. Also the flowering on avocados is interesting... There are type A flowers and type B flowers. Over a two day period, the type A flowers first open as a female for a few hours on day 1 and then open as a male flower for a few hours on day 2. The type B flowers do just the opposite: day 1 they open as males and day 2 they open as females. Even though all the flowers have functioning male and female parts, they are not self-pollinating. The ideal scenario for production is to have a type A avocado next to a type B avocado and hope for cross pollination. The short 2-day blooming period is also problematic... A windy day or a rainy day can either knock the blooms off or make the busy bees less busy... Also with biennial fruiting trees the theory is that the heavy fruiting year saps the energy of the tree so it requires the low-fruiting year to build back up for heavier bearing next year -- I'm not sure if that is true or not, but some think that is the reason.
I think we've grown all of our trees from seed which takes a little longer to fruit than grafted ones you might buy from a nursery so if those that have never produced are young, that may be why.
We've never really fertilized much, besides one year putting a good bit of horse manure around, but there are some well thought out fertilizer regimens for avocados you can find pretty easily by googling about.
I don't recall if you are in a cooler area of Costa Rica, but we are at 600m (18ºC-30ºC yearly temp range) and our bananas (plátanos, cuadrados, etc.) grow almost like a pesky weed. We have to thin them out a bit to prevent them from taking over an area after a few years. We usually cut off the male peduncle (the big dangly part) after the fruit have set on the theory that it makes for bigger fruit, but I don't know if that is an old wive's tale or not. Some of our cultivars regularly produce 10-12 "manos" (or layers as you say) and others maybe only 6 or 7. We've mostly been promoting plátanos enanos (dwarf plaintains) for the past couple of years and harvest those with maybe 4, 5, or 6 manos. Each mano might have 6 or 7 dedos (individual banana fruits.)
Sorry to hear about the off year. Hopefully the next will be bountiful!
--
Sam